Position of div blocks (simple css issue)

I have an issue in my project:

<div id="div1"></div>
<div id="div2"></div>

I do in css for desctop @media (min-width:768px) options display block and float left. For first with 70% for second 30%.
In mobile I need 100% width both of them and need to display div2 on top div1. Float right for first and float left for second not working. Need your help guys.